1991 Trek 6000 - shifting problem

[HerrOtto]

I replaced the chain with a generic new one, and it has greatly improved the shifting problem.

 

[The Spokemaster]

Try this -entirely loosen the small clamp at the rear derailleur that holds your derailleur cable inner wire.

Now, lift the rear wheel off the ground and turn the crank forward with your hand while pushing ( shifting ) the rear der. with your fingers -go up to the largest cog and then let go of the rear der. and see if the spring returns the chain cleanly back down to the smallest cog

If your rear der. actuates up and down with the inner wire not bolted in place = then you have some sort of cable / shiftlever 'issue'

 

[detroitbike]

Incorrect cable housings and/or missing
Cable end caps. These issues , especially In combination, can magnify shifting problems. For more precise shifting use compression-less housings. Instead of a spiral wound steel housing it is a series of wire strands run the length of the cable housing. Commonly referred to as SIS housing.

 

[Chad H]

One of the places where Suntour lost the game was on shifting set up. Suntour USA was right down the road in Kent Wa and knew the whole crew. Heck, I won a complete XC Pro get up when they launched the MT shifter in a tech promo in a quiz and my partner was Ned Overend! I raced on Suntour XC Pro and Comp and the old trick back in the early 90's was to switch out the upper pulley with a Shimano that had lateral "slop/movement". That pulley movement made it that it did not have to be 100% to be able to shift under load (horse shoes and hand grenades ya know). Good skill on the build, Chad
Oh, totally use SIS/Non-compression housing!